The present invention relates to a method of and apparatus for the treatment and purification of refuse, such as domestic refuse, industrial waste and the like, in particular of refuse/sewage sludge mixtures by the composting method, especially thermal retting.
Of the various ways of disposing of quantities of waste, that is to say of solid waste materials, such as domestic refuse, bulk waste, organic industrial by-products and industiral waste, also liquid waste, especially digested sludge from sewerage works, the composting process is to be preferred because in this way not inconsiderable quantities of the material can be recycled.
For the purpose of composting the waste material, or refuse/sludge mixture, the procedure is that the domestic and industrial waste with sewage sludge is reduced by milling action in suitably balanced quantities. Portions of the reduced mixture which exceed a predetermined dimension are then extracted by sifting, the materials concerned being glass, paper, plastic materials, rubber and metals. Small quantities of compostable materials in this diameter range may be returned to the milling stage. The through-material is then conveyed to a second finer sieve which accepts particles of predetermined dimension, for example under 10 mm, or possibly 15 mm. Fractions larger than the predetermined may again be returned to the milling stage, at which any iron is again extracted from the mass by magnets. When the through-material has reached the desired fineness, it is ready for composting. The proportion of sewage sludge is so adjusted that a relatively moist product is obtained which has a water content of approximately 45% to 55%.
The composting of waste material treated in the manner described still offers difficulty in obtaining complete and thorough purification by bedding the material. By the composting process an internal heat is generated in the material which may reach a temperature of approximately 75.degree. C., said heating being initiated by aerobic bacterial activity. The material is set up on clamps or stacks for composting. In this connection it has been found that the stacks cannot be built very high. Even with a height of approximately 1.30 m the so-called "black feet" occur, that is to say the material begins to rot because the air does not have sufficient access. Consequently, when waste material is stacked thus, it requires to be frequently turned over in order to ensure that the interior of the stack is also brought sufficiently into contact with the outer air. Turning over of the stack thus involves additional labor, time and expense. The turning over operation also has the disadvantage that heat is lost so that the desired temperature condition within the stack due to the aerobic bacterial activity requires time to build up. Even with heights of 1.30 m to 1150 m, a considerable physical area is required to cope with the daily accumulations of waste material. If stacks of lesser heights are employed, the space requirement is so enormous that composting becomes uneconomical. Attempts have also been made to ventilate the material, prepared for composting, by artificial means, that is to say, by utilizing fans which produce suction and pressure flows of air to aerate the stacks in order to prevent rotting of the waste material. Apart from the fact that the employment of blowers involves considerable additional costs and that these are required to be constantly moved from area to area, it is never possible to produce odourless composting of the material and prevent the occurrence of rotting patches in the stacks by this means. Artificial ventilating also cannot be so contrived that the bacterial activity in the compost heap proceeds undisturbed. Another attempt to employ natural aeration of the compost heap consists in placing the compost material in cages which are supported above the ground so that the surrounding air can reach the cage and consequently also the material from all sides. This method of aeration is not satisfactory either since unless the cages are rather small, pockets of rot occur which ruin the entire compost heap. In addition, a great deal of mechanical handling is required in filling emptying the cages, and also for turning over the material.